You say that “the stepper motor starts moving”, but can you please tell me if the XY movement of the machine is working correctly?
It is possible that the power supply is indeed producing a low voltage — if that is the case, then the power supply may indeed be failing (In your other message, you indicated that this is an older unit, and power supplies do sometimes fail), but then we would expect that the stepper motors are not moving correctly.
If the XY motion is working correctly, then it seems like the voltage sensor is not working correctly. If so, there is a hidden setting to disable the check, which will suppress the warning message. Please let me know what the situation is, and we’ll go from there.